She’s the mischievous little red-haired Swedish girl with the pigtails. Since 1945, this waif with no mother or father has rarely been out of the bestseller lists and continues to inspire musicals and movies. Heyday Films, the outfit behind Paddington and James Bond, is now developing an English-language adaptation of her stories.

What isn’t generally known outside her native Sweden are the circumstances in which author Astrid Lindgren created Pippi during the darkest period of the second world war, under the shadow of Hitler and Stalin.

“After I have worked so many years making the film, I am totally clear that Pippi is a child of the war. She never would have existed if there were not these terrible times,” Wilfried Hauke, the veteran German director behind new docudrama A World Gone Mad – The War Diaries of Astrid Lindgren (which will have its international premiere early next year) tells me.

His film features three generations of the Lindgren family – the author’s daughter, Karin Nyman, granddaughter, Annika Lindgren, and great-grandson, Johan Palmberg. It also has reconstructions in which leading Swedish stage actor Sofia Pekkari plays the author but her executors have made sure that every word she utters is taken exactly from what Lindgren actually wrote or said.

In 1939, Lindgren was living a quiet middle-class life in Stockholm. She was a housewife in her early 30s with two young children. Her husband, Sture, had a well-paid job at the Swedish National Association of Motorists, and didn’t spend much time at home.

Sweden may have been one of the few European nations to remain politically neutral during the second world war, but Lindgren was an ardent anti-Nazi and a news junkie. She scoured the papers, looking for accounts of the war, which she cut out and stuck in her notebooks. An excellent typist, she soon landed a secret wartime job at the postal control centre, steaming open and reading private and military letters.

From her letter reading, Lindgren realised that Jews were being sent to death camps. In the diaries she started keeping in this period, she frequently referred to the plight of wartime refugees.

Like many Swedes, Lindgren had grown up with a reverence for German literature, culture and philosophy – but now she was vetting private correspondence detailing Nazi atrocities. In May 1941, she writes that she has learned that 1,000 Jews a day are being “forcibly transported to Poland in the most shocking conditions … it is apparently Hitler’s intention to make Poland into one big ghetto where the poor Jews are to perish from hunger and squalor.”

“As long as you’re only reading about it in the paper you can sort of avoid believing it but when you read it in a letter … it suddenly brings it home, quite terrifyingly,” she added.

Hauke says: “You can see that the diary became more and more emotional. She felt this was now the beginning of the end of this old idea of European culture.”

Lindgren’s diaries consisted of 17 small handwritten schoolbooks. These were discovered in the Stockholm flat in which the author spent much of her life – and were published in 2015, more than a decade after her death. Hauke sees them as the forerunner to her children’s stories, which would not have existed without them.

The name “Pippi Longstocking” was coined by the author’s daughter, Karin. Lindgren invented tales about the character to distract Karin during her frequent wartime illnesses. Then, in 1944, when she hurt her ankle after a fall and was bedridden for three weeks, she began to work in earnest on writing and editing the first Pippi stories.

The creation of Pippi Longstocking was her way of dealing with the darkness seeping into her life. Her marriage was breaking down. Sture, she discovered, was seeing another woman and wanted a divorce. This was the “landslide that engulfed her existence”.

“But then she felt she had something of her own which was her writing. This made her strong again and helped her go through this crisis,” Hauke suggests.

One of her obsessions even then was children’s education – how, as the film-maker puts it, kids could be brought up “not to be psychopaths like Hitler or authoritarians, dictators and so on”. Bringing Pippi to life was part of this mission. The film-maker believes that “there was a lot of herself” in the character. Like her most famous fictional creation, she was a free-thinking individualist with endless resources of resilience and humour.

Lindgren’s great-grandson, Johan Palmberg, rights manager at the family company Astrid Lindgren Aktiebolag, was 11 years old when the author died aged 94 in 2002. He has very fond memories of her and tells me he still marvels at her instinctive way of communicating with children.

“She was very unique in that sense. It’s related to the reason why she was such a good children’s book author. She had this ability to go back in her own way to what things felt like as a child – and she could communicate with children on their own terms. She didn’t ask boring grownup questions. She asked interesting child questions.”

He believes that Pippi’s commercial appeal in 1945 lay in her cheery individualism. “The world had been in this terrible situation for many years and she comes as this fresh breath of air. She’s the antidote to the authoritarian regimes of Germany and the Soviets. She has all these characteristics of independence, free-thinking and kindness which is the antithesis to the Nazi ideology.”

Pippi’s 80th anniversary is now being marked with all the fanfare you might expect. “We’ve had birthday parties all year, more or less, and I think we are going to keep celebrating her because she is such an important character, especially with the world looking how it does,” the author’s great-grandson states. “Her independence, kindness and generosity are needed more than ever.”

• A World Gone Mad – The War Diaries of Astrid Lindgren is streaming on SVT in Sweden until 18 December, in German cinemas from 22 January and showing at the Copenhagen international documentary festival CPH:DOX from 13 to 22 March